The present invention relates generally to dimmable LED driving circuits and LED lamps including the same.
In conventional light fixtures using an incandescent bulb, dimming control functions are ordinarily performed by phase control of an AC power source, wherein the length of a conduction period is adjusted (i.e., a time duration of an AC waveform cycle) to adjust the amount of current supplied to the incandescent bulb, and thereby changing the magnitude of light output.
In recent years, light-emitting diodes (LED) that consume less power and have a longer life have become increasingly popular alternatives to incandescent bulbs. In some cases an LED lamp is desirably used in an existing light fixture intended for an incandescent bulb, but in such cases the demand remains for dimming control to be performed based upon phase control as described above.
As is known in the art, however, an LED and an incandescent bulb have different characteristics associated with a change in light output relative to change in current (i.e., light output versus current characteristics). While the light output vs. current characteristic of an LED is approximately linear, the light output vs. current characteristic of an incandescent bulb is nonlinear. Therefore, when dimming control based upon phase control (similar to that performed on an incandescent bulb) is performed on an LED, the magnitude of light output of the LED is different from that of the incandescent bulb even where the dimming rates are set the same, which may generate a strange and/or uncomfortable feeling. In particular, where an incandescent bulb and an LED are used in combination and lit simultaneously, the different light output vs. current characteristics are even more prominent.
Therefore, techniques for dimming control of an LED to exhibit a dimming characteristic similar to that of an incandescent bulb have been provided. Generally stated with regards to these techniques as are known in the art, current flow in an LED is controlled so as to provide a light output equivalent to that provided by an incandescent bulb in response to an equivalent external dimming control value.
For example, there has been proposed an LED driving circuit (i.e., LED lighting control means) which supplies in response to an input current from a power source (i.e., an AC constant current source) a reduced output current to an LED, to obtain a light output equivalent to that obtained when the output current is caused to flow without change in an incandescent bulb. Similarly, it has also been known to control current output to an LED based on a target current having a dimming curve similar to that of an incandescent bulb in response to the effective value of a phase-controlled AC voltage.
In configurations and techniques as conventionally known, however, it is required to determine or otherwise provide a formula for calculating current flow in an LED in advance from a relationship between the light output vs. current characteristic of an incandescent bulb and the light output vs. current characteristic of an LED. These driving circuits may include, for example, a program which calculates the target current in an LED according to the formula. Therefore, such a driving circuit generally becomes quite complex and expensive.